There are two principal technologies for producing pigment-grade titanium dioxide. The more recent is the Chloride Process in which titanium tetrachloride (TiCl4), produced by chlorination of synthetic rutile, is reacted with oxygen at about 1000° C. to give TiO2 in the rutile crystal form that is normally required for use as a pigment. In the older Sulphate Process ilmenite is treated with sulphuric acid to give titanyl sulphate (TiOSO4) which is hydrolysed to TiO2 in the anatase form. The anatase form must then be calcined to rutile at about 1000° C. The products from both processes are subjected to various proprietary finishing steps for ultimate use as pigments.
In the prior art, powders have been proposed in which mixtures of components include alumina and titania and which are applied by flame spraying to steel and other substrates to provide coatings that are highly impervious and abrasion, corrosion and oxidation resistant. For example, U.S. Pat. No. 3,607,343 discloses a wide range of spray powders which include complex mixtures of many oxides, including alumina and titania, as well as fluxing ceramics such as lithium cobaltite and thermally unstable resin varnishes. The mixed powders are applied through a high temperature spray gun to produce a continuous grey or black layer on a metal substrate. These coatings differ totally from conventional titania based pigments and from the titania coated alumina proposed here. The former are very fine white powders comprising only pure titania, while the latter is a fine white powder of alumina particles coated with titania.